许多短期对象的Java垃圾收集行为

Question

我有一个Java进程，它分配了许多非常短暂的对象（使用YourKit测量，平均年龄在10-20毫秒之间），并且很少有持久对象，它的功能是接受TCP消息，将其转换为某种方式，并生产卡夫卡。

大多数对象是由用Scala编写的Kafka生成器创建的，因此使用了大量不可变对象。 再次使用YourKit，我测量了来自Kafka的不可变对象的大小约为总数的75％，而来自Kafka的对象数量约为总数的53％。 绝大多数这些对象都没有超过20毫秒。

为了优化我的应用程序这个用例，我试图调整垃圾收集器，以便它将短期对象识别为常见用例，并在它到达旧代之前丢弃大多数。 这是我到目前为止使用的旋钮：

-verbose:gc -XX:+PrintGCDetails -XX:+PrintGCTimeStamps \
-XX:+PrintHeapAtGC -XX:+DisableExplicitGC -XX:+PrintTenuringDistribution \
-XX:+UseParallelOldGC -XX:NewRatio=1 \

最重要的标志是-XX:NewRatio和-XX:UseParallelOldGC但我注意到GC模式中的一些奇怪的行为。

以下是第一个完整GC之前的堆使用情况的说明：

以下是稳态堆使用情况的说明：

正如您所看到的那样，在第一个完整的GC之前，持续时间发生得非常缓慢（我也使用每个gc之后打印的堆信息确认了这一点），但是在第一个完整的GC之后，时效开始非常快。

我不明白这个的原因，我发现完全不需要完整的GC - 完整的GC几乎可以回收整个堆，因为绝大多数对象已经失去了强大的参考。

如果在每个GC之后包含堆信息会有所帮助，我会这样做，但是现在，这应该是足够的信息。

如果有人知道为什么会这样，请告诉我。 对于这样的应用来说，一小时的完整GC似乎太频繁了。

编辑：

根据要求，这里有一些额外的信息：

• 我使用的是Java 7，版本1.7.0_51。
• 我为此进程分配了1 GB的内存，并且堆已经固定为此大小。
• 在第一个完整GC之前，来自次要GC的一些堆输出：

 {Heap before GC invocations=395 (full 0): PSYoungGen total 521728K, used 519744K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 518656K, 100% used [0x00000000e0000000,0x00000000ffa80000,0x00000000ffa80000) from space 3072K, 35% used [0x00000000ffd00000,0x00000000ffe10000,0x0000000100000000) to space 2560K, 0% used [0x00000000ffa80000,0x00000000ffa80000,0x00000000ffd00000) ParOldGen total 524288K, used 177521K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 33% used [0x00000000c0000000,0x00000000cad5c560,0x00000000e0000000) PSPermGen total 33280K, used 33132K [0x00000000bae00000, 0x00000000bce80000, 0x00000000c0000000) object space 33280K, 99% used [0x00000000bae00000,0x00000000bce5b3d0,0x00000000bce80000) 35744.334: [GC Desired survivor size 2621440 bytes, new threshold 1 (max 15) [PSYoungGen: 519744K->1184K(521216K)] 697265K->179113K(1045504K), 0.0039310 secs] [Times: user=0.03 sys=0.00, real=0.01 secs] Heap after GC invocations=395 (full 0): PSYoungGen total 521216K, used 1184K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 518656K, 0% used [0x00000000e0000000,0x00000000e0000000,0x00000000ffa80000) from space 2560K, 46% used [0x00000000ffa80000,0x00000000ffba8000,0x00000000ffd00000) to space 2560K, 0% used [0x00000000ffd80000,0x00000000ffd80000,0x0000000100000000) ParOldGen total 524288K, used 177929K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 33% used [0x00000000c0000000,0x00000000cadc2560,0x00000000e0000000) PSPermGen total 33280K, used 33132K [0x00000000bae00000, 0x00000000bce80000, 0x00000000c0000000) object space 33280K, 99% used [0x00000000bae00000,0x00000000bce5b3d0,0x00000000bce80000) } 

• 第一个完整GC的堆输出：

 {Heap before GC invocations=1457 (full 0): PSYoungGen total 521728K, used 520064K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 519168K, 100% used [0x00000000e0000000,0x00000000ffb00000,0x00000000ffb00000) from space 2560K, 35% used [0x00000000ffd80000,0x00000000ffe60000,0x0000000100000000) to space 2560K, 0% used [0x00000000ffb00000,0x00000000ffb00000,0x00000000ffd80000) ParOldGen total 524288K, used 523194K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 99% used [0x00000000c0000000,0x00000000dfeeea68,0x00000000e0000000) PSPermGen total 34304K, used 34213K [0x00000000bae00000, 0x00000000bcf80000, 0x00000000c0000000) object space 34304K, 99% used [0x00000000bae00000,0x00000000bcf695e8,0x00000000bcf80000) 99997.829: [GC Desired survivor size 2621440 bytes, new threshold 1 (max 15) [PSYoungGen: 520064K->1600K(521728K)] 1043258K->525386K(1046016K), 0.0072540 secs] [Times: user=0.04 sys=0.01, real=0.01 secs] Heap after GC invocations=1457 (full 0): PSYoungGen total 521728K, used 1600K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 519168K, 0% used [0x00000000e0000000,0x00000000e0000000,0x00000000ffb00000) from space 2560K, 62% used [0x00000000ffb00000,0x00000000ffc90000,0x00000000ffd80000) to space 2560K, 0% used [0x00000000ffd80000,0x00000000ffd80000,0x0000000100000000) ParOldGen total 524288K, used 523786K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 99% used [0x00000000c0000000,0x00000000dff82a68,0x00000000e0000000) PSPermGen total 34304K, used 34213K [0x00000000bae00000, 0x00000000bcf80000, 0x00000000c0000000) object space 34304K, 99% used [0x00000000bae00000,0x00000000bcf695e8,0x00000000bcf80000) } {Heap before GC invocations=1458 (full 1): PSYoungGen total 521728K, used 1600K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 519168K, 0% used [0x00000000e0000000,0x00000000e0000000,0x00000000ffb00000) from space 2560K, 62% used [0x00000000ffb00000,0x00000000ffc90000,0x00000000ffd80000) to space 2560K, 0% used [0x00000000ffd80000,0x00000000ffd80000,0x0000000100000000) ParOldGen total 524288K, used 523786K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 99% used [0x00000000c0000000,0x00000000dff82a68,0x00000000e0000000) PSPermGen total 34304K, used 34213K [0x00000000bae00000, 0x00000000bcf80000, 0x00000000c0000000) object space 34304K, 99% used [0x00000000bae00000,0x00000000bcf695e8,0x00000000bcf80000) 99997.837: [Full GC [PSYoungGen: 1600K->0K(521728K)] [ParOldGen: 523786K->17123K(524288K)] 525386K->17123K(1046016K) [PSPermGen: 34213K->33877K(68096K)], 0.1576350 secs] [Times: user=0.53 sys=0.03, real=0.16 secs] Heap after GC invocations=1458 (full 1): PSYoungGen total 521728K, used 0K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 519168K, 0% used [0x00000000e0000000,0x00000000e0000000,0x00000000ffb00000) from space 2560K, 0% used [0x00000000ffb00000,0x00000000ffb00000,0x00000000ffd80000) to space 2560K, 0% used [0x00000000ffd80000,0x00000000ffd80000,0x0000000100000000) ParOldGen total 524288K, used 17123K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 3% used [0x00000000c0000000,0x00000000c10b8fc8,0x00000000e0000000) PSPermGen total 68096K, used 33877K [0x00000000bae00000, 0x00000000bf080000, 0x00000000c0000000) object space 68096K, 49% used [0x00000000bae00000,0x00000000bcf156f0,0x00000000bf080000) } 

• 第一次完整GC发生后，次要集合的堆输出：

  {Heap before GC invocations=1837 (full 9): PSYoungGen total 509440K, used 507104K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 494592K, 100% used [0x00000000e0000000,0x00000000fe300000,0x00000000fe300000) from space 14848K, 84% used [0x00000000fe300000,0x00000000fef38000,0x00000000ff180000) to space 14848K, 0% used [0x00000000ff180000,0x00000000ff180000,0x0000000100000000) ParOldGen total 524288K, used 342941K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 65% used [0x00000000c0000000,0x00000000d4ee7520,0x00000000e0000000) PSPermGen total 54272K, used 33876K [0x00000000bae00000, 0x00000000be300000, 0x00000000c0000000) object space 54272K, 62% used [0x00000000bae00000,0x00000000bcf15378,0x00000000be300000) 133247.303: [GC Desired survivor size 15204352 bytes, new threshold 1 (max 15) [PSYoungGen: 507104K->13696K(509440K)] 850045K->369421K(1033728K), 0.0318090 secs] [Times: user=0.37 sys=0.01, real=0.03 secs] Heap after GC invocations=1837 (full 9): PSYoungGen total 509440K, used 13696K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 494592K, 0% used [0x00000000e0000000,0x00000000e0000000,0x00000000fe300000) from space 14848K, 92% used [0x00000000ff180000,0x00000000ffee0000,0x0000000100000000) to space 14848K, 0% used [0x00000000fe300000,0x00000000fe300000,0x00000000ff180000) ParOldGen total 524288K, used 355725K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 67% used [0x00000000c0000000,0x00000000d5b63520,0x00000000e0000000) PSPermGen total 54272K, used 33876K [0x00000000bae00000, 0x00000000be300000, 0x00000000c0000000) object space 54272K, 62% used [0x00000000bae00000,0x00000000bcf15378,0x00000000be300000) } 

• 完成第一个GC之后的堆输出：

 {Heap before GC invocations=1457 (full 0): PSYoungGen total 521728K, used 520064K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 519168K, 100% used [0x00000000e0000000,0x00000000ffb00000,0x00000000ffb00000) from space 2560K, 35% used [0x00000000ffd80000,0x00000000ffe60000,0x0000000100000000) to space 2560K, 0% used [0x00000000ffb00000,0x00000000ffb00000,0x00000000ffd80000) ParOldGen total 524288K, used 523194K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 99% used [0x00000000c0000000,0x00000000dfeeea68,0x00000000e0000000) PSPermGen total 34304K, used 34213K [0x00000000bae00000, 0x00000000bcf80000, 0x00000000c0000000) object space 34304K, 99% used [0x00000000bae00000,0x00000000bcf695e8,0x00000000bcf80000) 99997.829: [GC Desired survivor size 2621440 bytes, new threshold 1 (max 15) [PSYoungGen: 520064K->1600K(521728K)] 1043258K->525386K(1046016K), 0.0072540 secs] [Times: user=0.04 sys=0.01, real=0.01 secs] Heap after GC invocations=1457 (full 0): PSYoungGen total 521728K, used 1600K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 519168K, 0% used [0x00000000e0000000,0x00000000e0000000,0x00000000ffb00000) from space 2560K, 62% used [0x00000000ffb00000,0x00000000ffc90000,0x00000000ffd80000) to space 2560K, 0% used [0x00000000ffd80000,0x00000000ffd80000,0x0000000100000000) ParOldGen total 524288K, used 523786K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 99% used [0x00000000c0000000,0x00000000dff82a68,0x00000000e0000000) PSPermGen total 34304K, used 34213K [0x00000000bae00000, 0x00000000bcf80000, 0x00000000c0000000) object space 34304K, 99% used [0x00000000bae00000,0x00000000bcf695e8,0x00000000bcf80000) } {Heap before GC invocations=1458 (full 1): PSYoungGen total 521728K, used 1600K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 519168K, 0% used [0x00000000e0000000,0x00000000e0000000,0x00000000ffb00000) from space 2560K, 62% used [0x00000000ffb00000,0x00000000ffc90000,0x00000000ffd80000) to space 2560K, 0% used [0x00000000ffd80000,0x00000000ffd80000,0x0000000100000000) ParOldGen total 524288K, used 523786K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 99% used [0x00000000c0000000,0x00000000dff82a68,0x00000000e0000000) PSPermGen total 34304K, used 34213K [0x00000000bae00000, 0x00000000bcf80000, 0x00000000c0000000) object space 34304K, 99% used [0x00000000bae00000,0x00000000bcf695e8,0x00000000bcf80000) 99997.837: [Full GC [PSYoungGen: 1600K->0K(521728K)] [ParOldGen: 523786K->17123K(524288K)] 525386K->17123K(1046016K) [PSPermGen: 34213K->33877K(68096K)], 0.1576350 secs] [Times: user=0.53 sys=0.03, real=0.16 secs] Heap after GC invocations=1458 (full 1): PSYoungGen total 521728K, used 0K [0x00000000e0000000, 0x0000000100000000, 0x0000000100000000) eden space 519168K, 0% used [0x00000000e0000000,0x00000000e0000000,0x00000000ffb00000) from space 2560K, 0% used [0x00000000ffb00000,0x00000000ffb00000,0x00000000ffd80000) to space 2560K, 0% used [0x00000000ffd80000,0x00000000ffd80000,0x0000000100000000) ParOldGen total 524288K, used 17123K [0x00000000c0000000, 0x00000000e0000000, 0x00000000e0000000) object space 524288K, 3% used [0x00000000c0000000,0x00000000c10b8fc8,0x00000000e0000000) PSPermGen total 68096K, used 33877K [0x00000000bae00000, 0x00000000bf080000, 0x00000000c0000000) object space 68096K, 49% used [0x00000000bae00000,0x00000000bcf156f0,0x00000000bf080000) } 

Answer 1

日志中唯一的完整GC ^{[严重的是，如果没有屠宰它就过于难以发布日志吗？]}需要大约¹⁶⁰毫秒，小的GC需要30毫秒或更短时间。

您没有设置任何暂停时间目标。

完整的GC之间经过一个小时或更长时间 。

如果你认为完整的GC在某种程度上是“邪恶的”并且必须在所有费用下避免，那么你就错了。

它们对CMS来说很麻烦，因为使用CMS时，Full GCs是一种失败模式的单线程回退策略，在大堆上可能需要很长时间。

但是你正在使用Parallel Old Gen收集器，即小堆上的吞吐量收集器 ，其中一个完整的GC非常快，特别是当大多数旧的gen内容是垃圾时，不需要通过标记扫描算法访问。

考虑到你甚至没有指定暂停时间目标，或者尝试过的CMS向我建议你甚至没有明确的目标。

所以这里没有问题。